c - 为什么 pthread_join() 的第二个参数是 **，指向指针的指针

Question

我是使用pthread的新手，也不熟悉指向指针的指针。也许有人可以解释为什么pthread_join()的第二个论点是一个void **。为什么会这样设计。

int pthread_join(pthread_t thread, void **value_ptr);

Answer 1

要通过函数的参数返回值，您需要传入变量的地址以接收新值。

由于pthread_join()被设计为接收传递给pthread_exit()的指针值，这是一个void*，pthread_join()期望一个实际上属于void**类型的void*的地址。

例：

#include <stdlib.h> /* for EXIT_xxx macros */
#include <stdio.h> /* for printf() and perror() */
#include <pthread.h> 
void * tf(void * pv)
{
  int * a = pv;
  size_t index = 0;
  printf("tf(): a[%zu] = %dn", index , a[index]);
  ++index;
  pthread_exit(a + index); /* Return from tf() the address of a's 2nd element. 
                          a + 1 here is equivalent to &a[1]. */
}

int main(void)
{
  int a[2] = {42, 43};
  pthread_t pt;
  int result = pthread_create(&pt, NULL, tf, a); /* Pass to tf() the address of 
                                                    a's 1st element. a decays to 
                                                    something equivalent to &a[0]. */
  if (0 != result)
  {
    perror("pthread_create() failed");
    exit(EXIT_FAILURE);
  }
  {
    int * pi;
    size_t index = 0;
    {
      void * pv;
      result = pthread_join(pt, &pv); /* Pass in the address of a pointer-variable 
                                         pointing to where the value passed to 
                                         pthread_exit() should be written. */
      if (0 != result) 
      {
        perror("pthread_join() failed");
        exit(EXIT_FAILURE);
      }
      pi = pv;
    }
    ++index;
    printf("main(): a[%zu] = %dn", index, pi[0]);
  }
  return EXIT_SUCCESS;
}

上面的程序预计会打印：

tf(): a[0] = 42
main(): a[1] = 43